Process for the preparation of a fiber-reinforced carbon composite

ABSTRACT

A preformed yarn useful in forming fiber-reinforced carbon composite articles is disclosed which includes a core of a multiplicity of inorganic reinforcing fibers, a mixed powder provided in the interstices between the fibers and including a finely divided carbonaceous binder pitch and a finely divided coke, and a flexible sleeve formed of a thermoplastic resin and surrounding the core.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 078,978, filed July 29, 1987,now U.S. Pat. No. 4,772,502.

This invention relates to a preformed yarn useful in formingfiber-reinforced carbon composite articles.

Inorganic fiber-reinforced carbon composites such as carbonfiber-reinforced carbon composites (generally known as "C--Ccomposites") in which reinforcing fibers are dispersed within a matrixof carbon have excellent mechanical properties at high temperatures,such as a high bending strength and a high resistance to abrasion athigh temperatures, and are used in a wide variety of applications, forexample, for aerospace parts such as rocket nozzles, aerocraft partssuch as sliders of brakes, working machine parts such as dies of hightemperature hot press meachines, and as materials for atomic reactorparts.

One known method for the production of such C--C composites is aso-called CVD method in which a premold of carbon fibers having adesired shape is heated in a furnace to a high temperature while feedinga hydrocarbon gas to the furnace, so that the hydrocarbon is thermallycracked to form carbon which deposits on the surface of the premold. Aprocess is also known in which yarns or woven or non-woven fabrics ofcarbon fibers are shaped into a desired structure with the use of athermosetting resin binder such as a phenol resin or epoxy resin, theshaped body being subsequently heated in an inert gas atmosphere tocarbonize the resin.

The prior art techniques encounter problems because the C--C compositesobtained are not uniform in their physical and mechanical propertiessuch as bending strength and density and because the processes arecomplicated and time consuming.

The present invention has been made in consideration of the problemsinvolved in the conventional techniques and has its object to provide anovel preformed yarn which is useful as a precursor for fiber-reinforcedcarbon composites, which is excellent in workability and processabilityin the fabrication of composite articles and which gives highlyheat-resisting composite articles exhibiting high and uniform mechanicalstrengths.

In accordance with one aspect of the present invention, there isprovided a preformed yarn useful in forming composite articles,comprising:

a core of a multiplicity of inorganic reinforcing fibers;

a mixed powder provided in the interstices between said fibers andincluding a finely divided carbonaceous binder pitch and a finelydivided coke; and

a flexible sleeve formed of a thermoplastic resin and surrounding saidcore.

In another aspect the present invention provides a process for thepreparation of a preformed yarn useful in forming composite articles,comprising the steps of:

continuously passing a multiplicity of inorganic reinforcing fibersthrough a bed of a mixed powder including a finely divided carbonaceousbinder pitch and a finely divided coke so that the mixed powder is takenin the interstices between said fibers;

assembling said mixed powder-carrying fibers to form a tow with saidmixed powder being held between said fibers; and

extruding a thermoplastic resin over said tow to form a sleevesurrounding said tow.

The present invention will now be described in detail below withreference to the accompanying drawing, in which:

FIG. 1 is a cross section of a preformed yarn according to the presentinvention; and

FIG. 2 is a diagrammatic illustration of an apparatus suitable for thepreparation of the preformed yarn according to the present invention.

Referring first to FIG. 1, the reference numeral 10 designates inorganicreinforcing fibers, generally continuous fibers, constituting a core ortow. The reinforcing fibers 10 may be, for example, carbon fibers, SiCfibers, alumina fibers, glass fibers and mixtures thereof. Of these, theuse of carbon fibers is preferred. Flame proof fibers, infusible pitchfibers and surface-treated fibers thereof, which are precursors ofcarbon fibers, are also be suitably used for the purpose of the presentinvention.

Methods of the fabrication of carbon fibers are generally classifiedinto two groups according to the types of the starting materials. Thefirst method uses petroleum pitch or coal tar pitch as a precursor rawmaterial whereas the second method carbonizes natural or syntheticfibers used as a raw material. Carbon fibers obtained by both of thesemethods may be used in the present invention. In the first method, pitchis first treated to obtain spinnable pitch, spinning the pitch,rendering the spun fibers infusible and then carbonizing the infusiblefibers. For example, pitch is reformed to have a softening point of180°-300° C., spun into fibers at a temperature of 250°-300° C. andtreated with an oxidizing gas at 150°-300° C. to obtain infusible fiberswhich in turn are carbonized at 800°-2500° C. In the second method,organic fibers such as cellulose fibers and acrylic fibers are processedto have flame-resistant properties and are then carbonized.

The inorganic reinforcing fibers preferably have a filament deniernumber ranging from about 0.05 to about 600 and filament counts rangingfrom about 50 to about 300,000, more preferably a filament denier numberranging from about 0.25 to about 16 and filament counts ranging fromabout 100 to about 48,000.

The amount of the reinforcing fibers is preferably 5-70%, morepreferably 20-60% based on the total volume of the preformed yarn. Whenthe content of the reinforcing fibers is below 5% by volume, thecarbonized composite articles obtained therefrom tends to beunsatisfactory in bending strength. On the other hand when the amount ofthe reinforcing fibers exceeds 70% by volume, the fibers in thecarbonized composite articles tend to become low in mechanical strengthbecause of the lack of the binder.

Referring continuously to FIG. 1, in the interstices between thereinforcing fibers 10 are provided finely divided, mixed powder 20including finely divided carbonaceous binder pitch and finely dividedcoke.

The carbonaceous binder pitch is preferably petroleum or coal binderpitch, more preferably isotropic, latently anisotropic or anisotropicbinder pitch derived from petroleum or coal and having a melting pointof 60°-320° C., more preferably 180°-300° C., a quinoline insolublecontent of 0-80% by weight, more preferably 30-70% by weight, and avolatile matter content of 5-60% by weight, more preferably 10-30% byweight. As the petroleum or coal pitch, there may be mentioned pitchobtained by heating a petroleum-derived heavy hydrocarbon oil such as anatmospheric residue, a vacuum residue or a catalytic cracking residue ora coal-derived heavy hydrocarbon oil such as coal tar or a sand oil atan elevated temperature, generally 350°-500° C. Mesophase spheresobtained from such a petroleum or coal pitch and bulk mesophase obtainedfrom the mesophase spheres by growth and coalescence thereof may also besuitably used for the purpose of the present invention.

The carbonaceous binder pitch has preferably an average particlediameter of 0.5-60 μm, more preferably 3-20 μm. Binder pitch with anaverage particle diameter of less than 0.5 μm tends to loose fluidityand becomes difficult to uniformly fill the interstices of the inorganicreinforcing fibers. On the other hand, too large an average particlediameter in excess of 60 μm also causes difficulties in being uniformlydispersed into the void spaces between the reinforcing fibers.

The finely divided coke to be mixed with the carbonaceous binder pitchis preferably a substantially unsoftenable coke having a volatile mattercontent of 10% by weight or less, more preferably 2% or less. A volatilematter content of the coke above 10% by weight tends to cause theformation of crack in the carbonized articles obtained therefrom. Cokederived from petroleum or coal may be suitably used.

The average particle size of the coke is preferably in the range of 0.5to 30 μm, more preferably 1 to 20 μm. An average particle size of below0.5 μm tends to cause difficulties in uniformly distributing the cokeparticles within the interstices of reinforcing fibers due to lowfluidity. Too large an average particle size in excess of 30 μm, on theother hand, tends to injure the reinforcing fibers and to cause theformation of pores or cracks in the carbonized products.

The blending ratio by weight of the binder pitch to the coke varies withthe intended use of the carbonized articles to be produced, butgenerally in the range of 9:1 to 1:9. The blending ratio is preferably7:3 to 3:7 for reasons of capability of minimizing the size and numberof pores in the carbonized product and of reducing the occurrence ofcracking.

The mixed powder to be incorporated between the reinforcing fibers mayfurther contain metal powder capable of forming a metal carbide when thepreformed yarn is subjected to carbonization conditions. Examples ofsuch metal include Ti, Si, Fe, W and Mo. These metals form carbidesduring carbonization treatment of the yarn and serve to improve theabrasion resistance, hardness and mechanical strength of the carbonizedcomposite articles. The metal powder preferably has an average particlesize of in the range of 0.5 to 30 μm. The content of the metal powder ispreferably 0.5 to 50, more preferably 3-20% based on the total weight ofthe mixed powder.

Further, for the purpose of improving mechanical properties at hightemperature of the carbonized product, an additive such as a metal, aninorganic compound or a thermosetting resin may be incorporated into themixed powder. Examples of suitable metals include Cu, Al, Sn, Pb, Bi,Sb, Zn, Mg, Ag and Cu-Zn. Examples of inorganic compounds include SiC,Pb₃ O₄, CdO, Al₂ O₃, MgO, Fe₂ O₃, ZnO, Cr₂ O₃, CaCO₃ and BaSO₄. Examplesof thermosetting resins include a phenol resin, unsaturated polyesterresins, epoxy resins, silicone resins, aromatic hydrocarbon resins andurea resins. Such an additive has preferably an average particle size of0.5 to 30 μm. The amount of the additive to be blended into the mixedpowder is preferably 0.5-50%, more preferably 3-20% based on the totalweight of the mixed powder.

As shown in FIG. 1, the tow or core of a multiplicity of the reinforcingfibers 10 holding therebetween the mixed powder 20 is covered with asleeve or sheath 30 formed of a thermoplastic resin.

The sleeve 30 is preferably formed of an easily decomposable andvaporizable, low softening point thermoplastic resin such as apolyamide, a polyethylene, a polypropylene, a polyester orpolyvinylidene fluoride. Above all, the use of a polyethylene orpolypropylene is preferred for reasons of inexpensiveness and capabilityof forming a thin sleeve. The thickness of the sleeve 30 is notspecifically limited. However, as long as the workability orprocessability of the preformed yarn is not adversely affected, the useof a thinner sleeve is more preferred for improving the properties ofcomposite articles. The use of a sleeve with a thickness of 7-30 μm isrecommended. Because of the presence of the sleeve 30, the core of thefibers 10 and mixed powder 20 is protected from inclusion of impuritiesor moisture therein and from being damaged or fuzzed. It is preferredthat the sleeve 30 be in close contact or shrunk fit with the core so asto minimize the void space between the core and sleeve and within thecore. Such a close contact is desirable for minimizing the inclusion ofair in the preformed yarn and, therefore, minimizing the occurrence ofpores within the final composite articles.

The process for the fabrication of the above-described preformed yarnwill now be described below with reference to FIG. 2. A bundle ofcontinuous reinforcing fibers 10 wound on a bobbin (not shown) iscontinuously fed through an unwinding equipment 1 to a chamber 2containing a mass of a mixed powder 20 including binder pitch and coke.An agitator 8 is disposed in the chamber 2 for mixing the powder 20. Themixing of the powder 20 may also be effected by feeding a fluidizing gassuch as air or nitrogen from the bottom of the chamber 2 so as tomaintain the mixed powder 20 in a fluidized state. The bundle of thefibers 10 is spread or loosened by any suitable means such as airinjection device or rollers and the spread fibers 10 are brought intocontact with the mixed powder 20 so that the mixed powder 20 is takenbetween the fibers 10.

The fibers 10 holding the mixed powder therebetween are then assembledinto a core or tow which is then covered with a sleeve in a sleeveforming device composed of a cross head 4 and an extruder 5. Designatedas 3 is a vacuum pump connected to the cross head 4 for shrinkinglyfitting the sleeve over the core and for providing tight bonding betweenthe sleeve and the core.

The preformed yarn thus prepared. is then cooled in a cooling zone (notshown) and wound on a take-up roller 7. Designated as 6 is a feed rollerfor drawing the preformed yarn at a constant speed.

If desired, the tow provided with the sleeve therearound may beprocessed, before cooling, by means of a stamping or knot forming deviceto form a plurality of axially spaced apart, thin, annular, depressedportions on the outer periphery of the sleeve by radially inwardlypressing, with heating, the preformed yarn so as to tighten the sleeveand the core together. The provision of the knots is desirable in aninstance where the preformed yarn is cut into a desired length, sinceescape of the mixed powder from the yarn can be minimized.

The preformed yarn according to the present invention finds utility in,for example, the production of inorganic fiber-reinforced compositearticles which are to be used under high temperature and high mechanicalstress conditions. Such composite articles may be easily prepared by,for example, filament winding or hot pressing in a simple manner.

When the preformed yarn according to the present invention is subjectedto hot pressing at a temperature of 400° C. or more, especially 600° C.or more, the shaped body produced is almost free of volatile mattersand, therefore, the succeeding carbonizing or graphatizing treatment canbe performed without encountering problems of generation of gases. Thus,the composite articles obtained with the use of the preformed yarns ofthis invention is substantially free of pores. Moreover, since thebinder pitch powder and coke powder are homogeneously distributed in theinterstices of the reinforcing fibers (in other words, the reinforcingfibers are homogeneously dispersed in the matrix of binder pitch andcoke powder), the composite articles obtained therefrom are uniform inphysical properties. The preformed yarn is also very suited for theproduction of composite articles having complex shapes and small radiiof curvature.

The following examples will further illustrate the present invention.

EXAMPLE 1

A bundle of carbon fibers having a filament counts of 3000, a filamentdiameter of 10 μm, a tensile strength of 310 kg/mm², a modulus of 22×10³kg/mm² and an elongation of 1.4% was continuously passed through a massof mixed powder at a rate of 50 m/min and a drawing tension of 30 g. Themixed powder was a 1:1 (by weight) mixture of a petroleum binder pitchhaving a particle size of 3-20 um, a softening point of 260° C., avolatile matter content of 30 weight % and a quinoline insoluble contentof 50 weight % and a coal-derived coke having a particle size of 3-10 μmand a volatile matter content of 1 weight %. The carbon fibers betweenwhich the mixed powder was held were then fed to a sleeve-forming crosshead connected to an extruder where a polyethylene was extruded over thecarbon fibers to form a sleeve therearound. The sleeve had a thicknessof 8 μm and an inner diameter of 2.5 mm. The thus obtained preformedyarn was wound around a bobbin at a rate of 50 m/min. The preformed yarnwas constituted of 58% by volume of the mixed powder, 34% by volume ofthe carbon fibers and 8% by volume of the sleeve.

The yarn was woven to form uni-directional sheets and 12 sheets of thewoven sheets are superimposed on one another and subjected to hot pressat a temperature of 600° C., a pressure of 500 kg/cm² and a pressingtime of 20 minutes to obtain a composite material having a bendingstrength of 1500 kg/cm², a density of 1.58 g/cm³ and a fiber content of40% by volume. The composite material was then calcined at 1500° C. for30 minutes to obtain a C--C composite having a bending strength of 1300kg/cm² and a density of 1.80 g/cm³.

EXAMPLE 2

The preformed yarn obtained in Example 1 was woven by means of a rapierloom to obtain a plain weave fabric. 20 Sheets of the fabrics weresuperimposed and subjected to hot press at a temperature of 600° C., apressure of 500 kg/cm² and a pressing time of 20 minutes to obtain acomposite material having a bending strength of 1000 kg/cm² and adensity of 1.65 g/cm³.

EXAMPLE 3

Example 1 was repeated in the same manner as described except that asilicon metal powder with a particle size of 3-10 μm was added to themixed powder in an amount of 5% based on the weight of the coke powder.A composite material having a bending strength of 1720 kg/cm² and adensity of 1.61 g/cm³ was obtained. The composite material was thencalcined at 1600° C. for 30 minutes in the atmosphere of nitrogen toobtain a C--C composite having a bending strength of 1690 kg/cm² and adensity of 1.83 g/cm³.

COMPARATIVE EXAMPLE 1

The 1:1 (by weight) mixture of the binder pitch and coke used in Example1 was heated to 370° C. to obtain a melt in which a plain weave carbonfiber fabric was immersed for impregnation. 20 Sheets of the thustreated fabrics having a fiber content of 40% by volume weresuperimposed and subjected to hot press in the same manner as in Example1 to obtain a composite material having a bending strength of 410 kg/cm²and a density of 1.60 g/cm³.

COMPARATIVE EXAMPLE 2

The 1:1 (by weight) mixture of the binder pitch and coke used in Example1 was scattered over plain weave carbon fiber fabric. Another sheet ofthe fabric was then superimposed on the mixed powder layer, on whichanother mixed powder layer was provided. The same procedure was repeatedto obtain a laminate composed of alternately superimposed 20 sheets ofcarbon fiber fabrics and 20 layers of the mixed powder and having acarbon fiber content of 40% by volume. The laminate was subjected to hotpress in the same manner as in Example 1 to obtain a composite materialhaving a bending strength of 210 kg/cm² and a density of 1.60 g/cm³.

COMPARATIVE EXAMPLE 3

Example 2 was repeated in the same manner as described except thatnylon-6 (thermoplastic resin) having a particle size of 20-40 μm wassubstituted for the mixed powder. The composite material obtained by thehot press was crumbly due to the decomposition of the thermoplasticresin during the hot pressing.

COMPARATIVE EXAMPLE 4

Example 2 was repeated in the same manner as described except that thebinder pitch powder alone was used in place of the mixed powder. Thecomposite material obtained as a result of the hot press had a bendingstrength of 450 kg/cm², a density of 1.58 g/cm³ and a carbon fibercontent of 45% by volume. Cracks were observed in the compositematerial.

COMPARATIVE EXAMPLE 5

Example 2 was repeated in the same manner as described except that thecoke powder alone was used in place of the mixed powder. Hot press ofthe fabric failed to give a composite material.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all the changes which come within the meaning and rangeof equivalency of the claims are therefore intended to be embracedtherein.

We claim:
 1. A process for the preparation of a fiber reinforced carboncomposite, comprising the steps of: continuously passing a multiplicityof inorganic reinforcing fibers through a bed of a mixed powderincluding a finely divided carbonaceous binder pitch and a finelydivided coke so that the mixed powder is taken in the intersticesbetween said fibers;assembling said mixed powder-carrying fibers to forma tow with said mixed powder being held between said fibers; extruding athermoplastic resin over said tow to form a sleeve surrounding said tow;and carbonizing the assembled tow, powder and sleeve by heating tothereby produce the fiber-reinforced carbon composite.
 2. A processaccording to claim 1 wherein said mixed powder further includes a finelydivided metal powder which forms a metal carbide when said coke andpitch are carbonized.
 3. A process according to claim 1, wherein saidextruding step is carried out while maintaining said tow under reducedpressures so as to substantially reduce void spaces between the fibersand between the tow and said sleeve.
 4. A process according to claim 1,further comprising radially inwardly hot-pressing the sleeve after saidextruding step to form a plurality of longitudinally spaced apart, thin,annular, depressed portions on the periphery of said sleeve so that saidsleeve and said core are tightened together at the hot-pressed portions.5. The process of claim 1 wherein said inorganic reinforcing fibers arecarbon fibers.
 6. The process of claim 1 further comprising passing afluidized gas through said bed to maintain said bed in a fluidizedstate.